1. Field of the Invention
The disclosures herein generally relate to a surface emitting laser array element, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
A surface emitting laser array element may be created by integrating plural surface emitting lasers (VCSEL: Vertical Cavity Surface Emitting Lasers). For example, the surface emitting laser array element may be created by forming a semiconductor layer stack by sequentially layering an n-type semiconductor multilayer film reflector, a lower spacer layer, a multiple quantum well active layer, an upper spacer layer, and a p-type semiconductor multilayer reflector on a n-GaAs substrate; forming plural mesas by etching the semiconductor layer stack in the vertical direction; forming an inter-layer insulating film; removing portions of the inter-layer insulating film arranged on the upper faces of the mesas to form openings corresponding to light emitting surfaces; and configuring the light emitting surfaces to emit laser light. It is noted that in such a surface emitting laser array element, upper electrodes are formed on the upper faces of the mesas, and the upper electrodes are connected to electrode pads by wirings that are formed on the inter-layer insulating film. Also, lower electrodes are formed on the rear face of the n-GaAs substrate.
In such a surface emitting laser array element, a current-confined structure is created by forming a semiconductor layer corresponding to a current-confined layer and performing selective oxidization for oxidizing the surrounding area of the current-confined layer. Specifically, to induce crystal growth of the semiconductor layer stack, an AlAs layer (or an AlGaAs layer with an Al composition ratio that is close to 1) as the current-confined layer is formed as part of the upper multilayer film reflector (DBR: Distributed Bragg Reflector) and the mesas are formed so that the side face of the current-confined layer is exposed. Then, steam oxidation is performed to oxidize a part of the current-confined layer from its side face to form a selectively oxidized region made of AlxOy. In this way, the AlAs region that is not oxidized forms a current-confined window (OA: Oxide Aperture) corresponding to a current-confined region.
The current-confined layer may be an AlxGayAsz layer containing a small amount of Ga. However, the oxidation speed of the AlxGayAsz layer largely depends on the amount of Ga contained. Thus, in the case of using AxGayAsz, it may be difficult to control the size of the current-confined region formed at the current-confined layer so that there may be variations in the laser characteristics of the surface emitting lasers and the throughput may be low due to the low oxidation speed. Accordingly, AlAs is preferably used as the material of the current-confined layer.
As can be appreciated from above, in a surface emitting laser having a current-confined structure, the production yield of the surface emitting layer may be improved and the throughput may be improved to thereby reduce manufacturing costs by using AlAs as the current-confined layer.
On the other hand, the current-confined window corresponding to the current-confined region is preferably arranged into a quadrangular shape so that the deflection direction may be controlled. However, the oxidation speed of the AlAs layer is largely dependent on its surface orientation. Thus, to arrange the current-confined region into a quadrangular shape, the mesas need to be arranged into quadrangular prisms or trapezoidal prisms.
The surface emitting laser array element is created by integrating the above surface emitting lasers in a two-dimensional array. For example, surface emitting lasers with mesas of approximately 30 μm2 may be arranged two-dimensionally at a pitch of approximately 40 μm to manufacture a densely integrated two-dimensional surface emitting laser array element. By using the surface emitting laser array element manufactured in this manner in the light source of an image forming apparatus such as a laser printer, a high-quality image equivalent to that obtained in offset printing may be output. The surface emitting laser array element may be superior to the edge emitting laser in that it can have plural surface emitting lasers that act as light emitting channels densely integrated into a two-dimensional array.
The mesa forming the surface emitting laser may be created by forming a resist pattern at the region where the mesa is to be formed by applying a photo resist on the semiconductor layer stack, exposing the photo resist with an exposure device, and developing the resist pattern; and then performing dry etching such as PIE (Reactive Ion Etching) on the semiconductor layer stack using the resist pattern as a mask. The above process may be used even in the case of fabricating a densely integrated two-dimensional surface emitting layer array element where the pitch of the surface emitting lasers is no more than 40 μm, for example.
It is noted that reactive ion etching may be advantageous in that it enables microfabrication in the micrometer scale and etching endpoint control in the nanometer scale. However, in this etching technique, the etching rate at the center portion is faster than the etching rate at the peripheral portion so that side etching and over etching may occur at the mesa located at the peripheral portion of the semiconductor layer stack. As a result, variations may be created in the laser characteristics of the surface emitting lasers. Such a problem may be particularly prominent with respect to the surface emitting lasers formed at the outermost region. It is believed that this problem occurs due to the fact that the mesas at the center portion are surrounded by adjacent mesas whereas the mesas at the outer periphery has portions that are not adjacent to any other mesa so that the ratio of the reactive ion to the by-product at these mesas at the outer periphery varies from the other mesas thereby creating variations in the etching rates.
In this respect, Japanese Laid-Open Patent Publication No. 2000-114656 (Patent Document 1) discloses a technique that involves arranging dummy mesas that are not actually used as lasers around mesas used for laser oscillation in order to prevent variations in the etching rate when forming the mesas and to thereby stabilize the process.
It is noted that the AlAs layer corresponding to the current-confined layer formed at the surface emitting laser having the current-confined structure is rather fragile so that it may easily break when pressure is applied thereon. Also, in the case where accommodations are made to avoid breakage of the AlAs layer, the surface emitting laser manufacturing method may become complicated and manufacturing costs may be increased. As a result, advantages such as the high production yield and high throughput of the surface emitting laser having the current-confined structure may be compromised.
Specifically, in a case where a contact exposure device that is small and relatively inexpensive is used as the exposure device in a photolithography process when forming the resist pattern, the pressure of contact of the exposure device with the photo mask during exposure of the same may cause the AlAs layer to break. On the other hand, when the contact pressure of the exposure device is reduced to prevent breakage of the AlAs layer, adhesion of the photo mask may be reduced so that the transcriptional precision of the lithography process may be degraded and the production yield and uniformity of the surface emitting lasers may be compromised, for example.
In this respect, since high transcriptional precision is required in fabricating a surface emitting laser array element having surface emitting lasers densely integrated into a two-dimensional array, the use of the reduced projection exposure method in the photolithography process may be contemplated. However, an exposure device for realizing reduced projection exposure is relatively large and expensive so that the cost for manufacturing the surface emitting lasers may be significantly increased in the case where such exposure device is used, for example.
On the other hand, desirable transcription characteristics may be obtained using the relatively small and inexpensive contact exposure device by arranging dummy mesas around the mesas that are to form the surface emitting lasers and dispersing the contact pressure of the exposure device so that adhesion of the photo mask to the substrate may be increased. That is, by arranging the dummy mesas, the densely integrated two-dimensional surface emitting laser array element with the current-confined structure may be manufactured at a relatively low cost. Japanese Laid-Open Patent Publication No. 2007-27362 (Patent Document 2) discloses such a technique for obtaining desirable transcription characteristics by arranging the dummy mesas and dispersing the contact pressure during contact exposure.
In the case of arranging dummy mesas at the densely integrated two-dimensional surface emitting laser array element around the region where the surface emitting lasers are to be formed, wirings have to be formed above the dummy mesas. However, in this case, the dummy mesas may become obstacles for the wirings because the wirings have to be arranged over the uneven surface created by the dummy mesas. That is, wiring disconnection may occur at the edge portions of the upper faces of the dummy mesas, or the film thickness of the wirings may be reduced to cause an increase in electrical resistance, for example.
The wiring may be created by depositing a metallic material through vacuum vapor deposition or sputtering to form a metal film, for example. In such case, the film thickness of the deposited wiring at the edge of the upper face of the mesa (edge of the concave mesa) and the side face of the mesa may be thinner than the other portions so that wiring disconnection may be prone to occur and the electric resistance of the wiring may be increased at these locations.
Also, in the case where the wiring is formed through vacuum vapor deposition or sputtering, the substrate is typically revolved around its center while the wiring is formed to ensure in-plane uniformity of its film thickness. However, in the case where the mesa has a quadrangular prism shape or a trapezoidal prism shape, the mesa shadows the substrate three-fourths of the film deposition time so that film deposition cannot be performed during this time. Thus, ring disconnection is more prone to occur in this case compared to a case where the mesa has a cylindrical shape or a conical shape.
In this respect, Patent Document 2 discloses forming dummy mesas that are divided into small sections so that the wiring does not have to be arranged over the dummy mesas.
Further, Japanese Laid-Open Patent Publication No. 2008-34637 (Patent Document 3), Japanese Laid-Open Patent Publication No. 2006-13366 (Patent Document 4), Japanese Laid-Open Patent Publication No. 2007-150170 (Patent Document 5), and Japanese Laid-Open Patent Publication No. 2005-191343 (Patent Document 6) disclose techniques for improving the step coverage of the wiring. Specifically, Patent Document 3 discloses covering the entire side face of the mesa with a p-side electrode wiring. Patent Documents 4 and 5 disclose arranging the mesa into a tapered structure in order to improve wiring coverage. Patent Document 6 discloses a planarization technique using polyimide.
Other known methods for improving the step coverage of the wiring formed through vacuum vapor deposition or sputtering include depositing the deposition particles from an oblique angle with respect to the substrate so that the wiring formed at the side wall of the mesa may be thickened.
However, the above disclosures still fail to resolve certain problems and disadvantages. Although Patent Document 1 discloses arranging dummy mesas that are not actually used as lasers around mesas used for laser oscillation to reduce variations in the etching rate for forming the mesas, as described above, in such a configuration, the dummy mesas may become obstacles for the electrode wiring when the mesas forming the surface emitting lasers are densely integrated into a two-dimensional array. That is, when the wiring is formed on the upper surfaces of the dummy mesas, wiring disconnection may occur at the edges of the upper faces of the dummy mesas and the wiring resistance may be increased as a result of the reduced thickness of the wiring.
Although Patent Document 2 discloses forming dummy mesas that are divided into small sections so that the wiring does not have to be arranged over the dummy mesas and the contact pressure may be dispersed by the dummy mesas, in such a configuration, because the shapes of the mesas that are to become the surface emitting lasers are different from the shapes of the dummy mesas, the etching rate during the dry etching process for forming the mesas may not be uniform. Thus, even if the dummy mesas are formed, the problem of variations in the etching rates may not be resolved.
Although Patent Document 3 discloses forming a p-side electrode wiring on all the side wall faces of the mesas to improve step coverage of the wiring, in such a configuration, internal stress of the wiring formed on the mesa side walls may act on the surface emitting lasers to cause lattice defects so that reliability of the surface emitting lasers may be degraded. Also, the deflection direction of emitted laser light may vary from the predetermined direction. Further, in arranging the surface emitting lasers into a two-dimensional array, wiring may be arranged between the mesas that form the surface emitting lasers. However, in the case where such technique is used, the space between the mesas cannot be reduced thereby posing an obstacle to densification of the surface emitting lasers.
Although Patent Documents 4 and 5 disclose arranging the mesas into tapered structures to improve the wiring coverage, such measures are inadequate and the film thickness of the wiring may still be reduced at the mesa side walls (step coverage may be inadequate) so that the occurrence of wiring disconnection and a decrease in reliability may not be effectively prevented.
Also, in the case where the planarization technique disclosed in Patent Document 6 is used, an extra process has to be performed to form the polyimide film so that manufacturing costs may be increased. Further, since the polyimide film comes into contact with the side face of the mesa, internal stress may be generated and the internal stress may act on the surface emitting laser to induce crystal defects and decreased reliability.
Also, in the case of depositing a metallic material from an oblique angle to form the wiring at the side face of the mesa to ensure adequate wiring coverage, a flash may be formed at the edge of the airing arranged at a bottom portion between the mesas. The flash formed at the wiring may turn into particles at a subsequent process and induce a short circuit between wirings. Further, in the case where the substrate is arranged to revolve around its center while deposition particles are deposited from an oblique angle with respect to the substrate in order to ensure uniform in-plane distribution of the wiring film thickness upon forming the wiring, if the mesas are arranged into quadrangular prism shapes, the mesas may shadow the substrate during three-fourths of the film deposition time to thereby prevent film formation during this time. Thus, it may be difficult to improve step coverage of the wiring in this case.